1. Field of the Invention
The present invention relates to the production of hydrogen cyanide by reacting methane, ammonia and oxygen in the presence of a catalyst. More particularly, the process of the present invention relates to the use of an oxygen enriched gas mixture to safely increase the production of hydrogen cyanide in an air based production unit while not sacrificing catalyst performance.
2. Brief Description of the Prior Art
Hydrogen cyanide is generally manufactured by the Andrussow process by reacting the feed streams of air, natural gas, and ammonia over precious metal catalysts in gauze form. A description is provided, for example, in the Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Volume 7, pages 753 to 782. Natural gas and NH.sub.3 are each usually fed in slight stoichiometric excess to air and some degree of preheat is generally used. The feed gases to the converter are thoroughly premixed. There are two important considerations to determine the process feed mixture composition.
First, the feed gas mixture is maintained above the upper flammability limit (UFL) to prevent the mixture from entering the explosive range. The flammability limits of individual combustible gases present in the reaction mixture can be determined by LeChatelier's empirical method:
Flammability Limit=1/.SIGMA.(f/L.sub.i), where
f=relative mole faction of flammable gases without oxygen or inert, and PA1 L.sub.i =flammability limit of individual flammable component i.
Using LeChatelier's formula, the upper flammability limit of a gas mixture can be calculated in air and in oxygen, and can be used to map the entire flammable domain for a particular gaseous mixture.
Second, the feed mixture is adjusted to maximize the selectivity of HCN. If insufficient air is provided, the catalyst can become inactivated by carbon deposits. However, increasing the air to (CH.sub.4 +NH.sub.3) mole ratio can move the gas mixture close to the upper flammability limit.
The overall reaction to form hydrogen cyanide is represented by EQU CH.sub.4 +NH.sub.3 +1.50.sub.2 .fwdarw.HCN+3H.sub.2 O
The reaction pressure is generally from 1 to 3 atm. and the temperature is generally from about 1100.degree. C. to 1200.degree. C. Lower temperatures may lead to carbon formation, which reduces reactivity and can cause mechanical break up of the gauze. The yield based on NH.sub.3 feed is typically above 60%. Unconverted NH.sub.3 is recovered from the product gas. Traces of iron contamination on the platinum catalyst reduces yield and hence promotes coke deposition. Platinum losses in the reaction are small because formation of volatile platinum oxides is less likely in the overall reducing environment.
The reactions taking place in the process are much more complex than represented by the overall reaction above. The oxygen (in the feed air stream) oxidizes a portion of the methane, making the overall reaction exothermic. Most of the heat required for hydrogen cyanide formation is supplied by combustion of methane. The reaction of methane with ammonia to form hydrogen cyanide is endothermic and produces hydrogen which is partially oxidized. However, the oxidation of the hydrogen is not complete so that the product gas also contains appreciable amounts of H.sub.2 and CO as well as unreacted NH.sub.3 and a small amount of CH.sub.4 in addition to N.sub.2 from the air.
Oxygen enrichment of air allows an increased throughput of ammonia and natural gas feed that leads to higher production capacity. However, enriching air with oxygen leads to a change in the ratios of combustibles, oxygen, and inerts thereby changing the upper flammability limit of the mixture. This requires an adjustment in process operating conditions of feed ratios. Further, the feed ratios must be adjusted to maximize the cyanide yield.
U.S. Pat. No. 3,104,945 discloses the use of oxygen enriched air in the production of hydrogen cyanide. The ratio of methane to ammonia in the initial gas mixture may vary in the range of 0.8:1 to 1.3:1 in the disclosed process. This means the ammonia-air ratio is from 0.1 to 0.26 or the ammonia-oxygen ratio is from 0.48 to 0.24 (when using 21% oxygen in air). The air to ammonia ratio ranges from 3.9:10. The preheat temperature in the gas mixture ranges from 400.degree. to 525.degree. C.
German Patent 1,282,209 also discloses the use of oxygen enriched air in the production of hydrogen cyanide. The patent further suggests that when using the enriched gas mixture, one should remain outside the flammability limits. The oxygen enriched levels are from 24.5 to 40% O.sub.2 /(O.sub.2 +N.sub.2) The lab scale experiments disclosed were run with a preheat temperature of 110.degree. C. and a catalyst temperature ranging from 1100.degree. to 1200.degree. C.
While the use of oxygen enrichment of air in the production of hydrogen cyanide has been disclosed, it is important to understand that oxygen enrichment of a gas mixture of air, ammonia and methane can bring the mixture closer to flammability limits if the volume percent of the combustibles (ammonia and methane) are not changed. Furthermore, oxygen enrichment can lead to increased catalyst temperature if the ammonia/oxygen and methane/oxygen ratios are not changed appropriately from the normal air based case. This can lead to adverse changes in the catalyst behavior relating to both catalyst life and conversions. Also, because of higher throughput with oxygen enrichment and higher temperatures at the catalyst gauze, a higher heat duty at the waste boiler would be required in the process. In many cases extra heat duty may not be available in the existing equipment, thereby leading to a bottleneck.
Therefore, a process which can increase the production of hydrogen cyanide by using oxygen enriched air, but which is also safe, efficient and effective, and does not adversely effect the catalyst while overcoming the foregoing problems would be useful in the industry. Such a process would allow one to take existing plants, optimizing production and capacity, as well as selectivity, but not sacrificing safety or the catalyst performance.
Accordingly, it is an object of the present invention to offer a process for the production of hydrogen cyanide with high production capacity, safe operation and continued excellent catalyst performance.
Another object of the present invention is to provide a process for the production of hydrogen cyanide which would allow one to begin with an existing plant for producing hydrogen cyanide and improving the hydrogen cyanide production safely and efficiently.
Yet another object of the present invention is to provide one with a safe process for producing hydrogen cyanide which involves the adjustment of the feed streams to the process.
These and other objects of the present invention will become apparent upon a review of the following specification and the claims appended thereto.